Lithium ion secondary batteries, which have small size and large capacity, have been widely used as power supplies for electronic devices such as mobile phones and notebook computers and have contributed to enhancing convenience of mobile IT devices. Attention is now drawn to use of such batteries in large-scale applications, for example, power sources for automobiles and the like, and power storage devices for smart grid.
It is urgent to further increase the energy density of lithium ion secondary batteries, and examples of the process to increase the energy density include a process of using active materials having large capacity, a process of increasing the operating potentials of batteries, and a process of enhancing the charge/discharge efficiency, cycle life and the like. Among these, the process of increasing the operating potentials of the battery is a measure effective for size and weight reduction of battery modules used in electric vehicles and the like because the process can provide fewer numbers of battery packs in series than conventional battery packs.
Materials having a 4-V class operating potential (average operating potential=3.6 to 3.8 V: versus the lithium potential) such as lithium cobalt oxide and lithium manganese oxide are used as positive electrode active materials for lithium ion secondary batteries. This is because the redox reaction of Co ions or Mn ions (Co3+<->Co4+ or Mn3+<->Mn4+) regulates a developed potential. In contrast, it is known that, for example, a compound in which Mn of spinel lithium manganese oxide is substituted with Ni, Co, Fe, Cu, Cr and the like as the active material presents a 5-V class operating potential (average operating potential=4.6 V or more: versus the lithium potential). As Mn exists in the tetravalent state in such a compound, the redox of the substituted element, instead of the redox reaction of Mn, regulates the operating potential.
LiNi0.5Mn1.5O4, for example, has capacity of 130 mAh/g or more, an average operating potential of 4.6 V or more versus Li metal, and can be expected as a material having a high energy density. Additionally, spinel lithium manganese oxide, which has three-dimensional paths for diffusing lithium, has advantages superior to other compounds, such as excellent thermodynamical stability, ease of synthesis, relatively inexpensive raw materials, and abundant resources.
In contrast, use of a 5-V class positive electrode notably causes problems due to decomposition of electrolyte solutions, such as gas evolution and a drop in capacity in long-term cycles and under high-temperature conditions. In order to prevent electrolyte solutions from decomposing, development of electrolyte solutions highly resistant to oxidation is proceeding.
Patent Literatures 1 and 2 describe a secondary battery in which fluorinated compounds, such as fluorinated ethers, fluorinated carbonate esters, fluorinated esters, fluorinated acrylates, and fluorinated cyclic carbonates are employed as the solvent when a positive electrode active material which exhibits a charge/discharge region of 4.5 V or more is employed.
Patent Literatures 4 and 5 describe a secondary battery in which an electrolyte solution containing a sulfone compound and a fluorinated solvent is employed.
As described above, examples of electrolyte solution containing fluorinated ethers or sulfone compounds have been already disclosed. However, Patent Literatures 1 and 2 describe no examples in which a fluorinated solvent and a sulfone compound are mixed. Patent Literature 3 exemplifies an electrolyte solution which is a combination of sulfolane and a carbonate ester, but does not describe fluorinated ethers. Patent Literatures 4 and 5 exemplify an electrolyte solution which is a mixture of a fluorinated ether and sulfolane, but describe no effect relating to e.g. suppression of gas evolved from decomposition of the electrolyte solution, which is a problem inherent to 5-V class positive electrodes, because all the positive electrodes employed therein are 4-V class positive electrodes such as lithium cobalt oxide.